A large number of light-sensitive materials comprising a support having thereon a light-sensitive layer are known, where the image formation is performed by imagewise exposing the light-sensitive material. Of these, a technique of forming an image by heat development is a system capable of satisfying the issue of environmental conservation or simplifying the image formation means.
In recent years, reduction of the amount of waste processing solutions is keenly demanded in the field of photomechanical process from the standpoint of environmental conservation and space savings. To cope with this, techniques are required to produce light-sensitive heat-developable materials for use in photomechanical process, which can be effectively exposed by a laser scanner or laser image setter and can form a clear black image having high resolution and sharpness. Such light-sensitive heat-developable materials can provide to users a heat development processing system being dispensable with use of solution-type processing chemicals, simple and freed from incurring environmental destruction.
Methods for forming an image by heat development are described, for example, in U.S. Pat. Nos. 3,152,904 and 3,457,075 and D. Morgan and B. Shely, Imaging Processes and Materials, "Thermally Processed Silver Systems" A, 8th ed., page 2, compiled by Sturge, V. Walworth and A. Shepp, Neblette (1969). The light-sensitive material used contains a light-insensitive silver source (e.g., organic silver salt) capable of reduction, a photocatalyst (e.g., silver halide) in a catalytic activity amount, and a reducing agent for silver, which are usually dispersed in an organic binder matrix. This light-sensitive material is stable at room temperature. However, when it is heated at a high temperature (e.g., 80.degree. C. or higher) after the exposure, silver is produced through an oxidation-reduction reaction between the silver source (which functions as an oxidizing agent) capable of reduction and the reducing agent. The oxidation-reduction reaction is accelerated by the catalytic action of a latent image generated upon exposure. The silver produced by the reaction of the silver salt capable of reduction in the exposure region provides a black image and this presents a contrast to the non-exposure region. Thus, an image is formed.
This type of heat-developable light-sensitive material has been heretofore known but in many of such light-sensitive materials, the light-sensitive layer is formed by coating a coating solution using an organic solvent such as toluene, methyl ethyl ketone or methanol, as a solvent. However, use of an organic solvent as a solvent is not preferred because of its adverse effect on a human body during the production process, and organic gas emission, which may be a cause of global warming, or in view of the cost for recovery of the solvent, requirement for explosion protection facilities or the like.
These problems may be overcome by using water as an application solvent (application scheme utilizing water as an application solvent will be referred to as "aqueous application" hereinafter). For example, JP-A-49-52626 (the code "JP-A" as used herein means an "unexamined published Japanese patent application"), JP-A-53-116144 and the like disclose use of a gelatin binder. JP-A-50-151138 discloses use of polyvinyl alcohol as a binder.
However, such use of water-soluble binders leads to simultaneous dehydration shrinkage and thermal expansion of the binders during the heat development, and these phenomena produce corrugates of films because their degrees are different from that of thermal expansion of supports. Thus, the use exclusively produce films unsuitable for color printing, wherein the films are laminated for use.
This problem may be solved by using a polymer latex. For example, WO97/4355, JP-A-8-137045 and the like disclose the production of heat-developable image-recording materials through aqueous application by utilizing a polymer latex as a binder.
However, in order to form uniform image-forming layer and protective layer without impairing photographic properties, it is necessary to use a polymer latex application solution having a low MFT (minimum film-forming temperature), and for this, it is essential to form an applied film at an appropriate MFT by utilizing a polymer latex and/or film-forming aid having a low Tg (glass transition temperature). However, a lowered MFT affords a softer applied film after application and drying, and such a film is likely to suffer problems. For example, such a film may adhere to members of heat-developing apparatus (e.g., transportation rollers, guide panels etc.) to cause transportation error or become likely to have scratches. Further, correction solutions containing an organic solvent as a dissolution medium are often used for correction of pinholes in images after the heat development. Such correction solutions may dissolve or greatly swell applied films of corrected portions to degrade images.
Various crosslinking agents have generally been used to crosslink polymers aiming at improvements of heat resistance, durability, mechanical properties and the like of the polymers. In general, these techniques often use a high crosslinking reaction temperature. Therefore, they may cause problems, for example, they cause high fogging, and make it difficult to obtain high contrast, in particular, when a nucleating agent is used in order to obtain high contrast photographic properties. In addition, many of crosslinking agents are reactive with active hydrogen (e.g., epoxy groups), and even when a crosslinking agent is added to a protective layer, it may be transferred to an image-forming layer by diffusion. The crosslinking agent transferred in such a manner may disadvantageously react also with reagents necessary for the image formation to degrade photographic performance. Therefore, there has been desired a heat-developable image-recording material that allows crosslinking without degrading photographic properties, and exhibits excellent suitability for heat development or suitability for image correction after the heat development.